The invention pertains to a scissors lifting table device having a pair of scissors positioned between a carrier unit and a base unit. The scissors have an inner arm and an outer arm pivotably connected to one another at a scissors axle. The lifting device further includes a lifting truck movable back and forth by means of a drive for opening and closing the scissors, a drive shaft coupled to the drive and movable in both rotational directions, and at least one traction means looped around the drive shaft and coupled to the lifting truck.
A lifting table device of this type, typically used for raising and lowering loads, e.g., in automobile production, is described in U.S. Pat. No. 3,785,462, which discloses a scissors lifting table which uses a cable with one end anchored to the carrier unit and the other end fixed to a lower cam shaft. The cable is looped around several rollers, as well as a roller-like lifting element that is positioned between the inner and outer arms and movable towards the scissors axle. By winding and unwinding the cable on the lower cam shaft by means of a shaft powered by a chain, the lifting element is moved relative to the scissors axle, so that the carrier unit is raised or lowered.
Another scissors lifting table is described in DE 90 05 566 U1. In this scissors lifting table, two pairs of scissors are arranged parallel to one another at opposite sides of a carrier device having a platform and a frame-like base unit, and are provided for raising and lowering the platform by opening and closing. Opening and closing the scissors is accomplished by the back and forth movement along the base unit of a lifting slide or truck with laterally located oblique lifting curves on its upper side which contact rollers located near the scissors axle. The movement of the lifting slide is effected by a threaded spindle positioned perpendicular to the scissors axle. Such a spindle is a precision part and is typically bearing-mounted in a spindle nut. Such a spindle drive is not only costly but is also sensitive to transverse forces and vibrations which can disturb operation or damage the spindle drive.
In the scissors lifting table disclosed by DE 44 13 527 A1, an actuating drive with essentially horizontal rollers is located at the level of the rotational axis of the scissors. The rollers work in combination with control curves situated on the scissors arms, while the shafts of the rollers are parallelly displaced in the medial plane by an actuating drive. A hydraulic reciprocating piston actuator acts as the actuating drive. Such a hydraulic actuator typically involves jerky starting and stopping of the lifting movement and can also cause oily deposits and is therefore undesirable.
Additional scissors lifting tables are described in DE 83 29 409 U1 and DE 197 44 519 C1.
Disclosed in DE 33 31 872 A is a roller-like lifting device which has a piston-cylinder unit connected to a chain which is fastened to a roller. The roller is positioned near the pivot of the scissors arms and is pulled toward the pivot by movement of the piston so as to raise and lower the upper part.
Fundamental to the invention is the objective of providing a scissors lifting table device of the type cited in the introduction, in which the lifting movement can be controllably and reliably performed without the risk of dirt accumulation.
The objective is realized by the inventive scissors lifting device which utilizes at least one band-like toothed belt. The drive shaft has on its circumference, at least in some areas, a toothing which works in combination with the toothing of the one or more toothed belts.
Using this configuration of the drive mechanism of the lifting truck allows for controlled and reliable movement of the lifting truck without risk of disruption and avoids the undesirable accumulation of dirt, while being cost-effective. In a preferred embodiment several, e.g., four, toothed belts are used in parallel, which is readily possible with the drive shaft. Thus, when a belt is damaged the lifting operation can be safely continued until a suitable repair opportunity is available.
In a preferred embodiment, the drive shaft is rigidly mounted on the base unit and at least one reversal element is provided. The reversal elements being rotatably mounted on a rotational shaft at a fixed distance from and parallel to the drive shaft. The reversal elements redirect the belt during use.
In preferred operation the reliability of the drive mechanism is ensured by the use of several, e.g., three or four, toothed belts as well as a corresponding number of reversal elements.
The toothed belt(s) preferably forms an upper part and a lower part and the lifting truck is preferably coupled to the upper part so that movement of the lifting truck is facilitated.
The preferred toothed belt has two ends, both of which are fastened to the lifting truck, while a tensioning device for tightening the respective toothed belt is preferably interconnected between at least one of the ends and the truck. Alternatively, tensioning of the toothed belt could be accomplished by adjusting the respective reversal elements.
For the installation and maintenance of the scissors lifting table, it is preferable that the drive have a transmission section with a tubular shaft stub, into which a matching end section of the drive shaft is inserted.
To further simplify maintenance the drive is preferably mounted laterally outside the base unit and fixed to the base unit. The drive is preferably equipped with a brake and a torque support.
In a preferred design two parallel inner arms are mounted on inner surfaces of the carrier unit and two parallel outer arms are mounted on the outer side of the carrier unit and the lifting truck rests on the inner arms so as to roll or glide, while an interchangeable lifting curve or cam that determines the lifting movement is provided on at least one outer arm for opening and closing the scissors to effect the lifting movement.
It is preferred that the drive shaft be located near a base-side pivoting axis of the inner arms and the rotational shaft of the reversal elements be located near the inner arms on the side of the scissors axle opposite the drive shaft, then the lifting truck is safely guided in the inner arms to complete the lifting movement all the way to the open and closed positions.
It is also preferred that the lifting truck have two pairs of rollers, staggered in the running direction and supported on the inner arms, and a pair of lifting rollers for working in combination with the lifting curves or cams.
An alternative design of the scissors lifting table device comprises a lifting truck glidingly mounted on the base unit and bearing on both sides a pair of lifting curves, oriented in the movement direction, which work in combination with a pair of lifting rollers riding thereon that are rigidly mounted on two opposing parallel arms of the scissors or the scissors axle.
The invention is explained in greater detail below with reference to an embodiment example illustrated in the appended drawings.